1. Field of the invention
The present invention relates to a hot melt glue applicator and a glue stick for use therein.
2. Description of the Related Art
FIG. 1 of the appended drawings shows a hot melt glue applicator of a type that is widely used at present. It is in the form of a gun with a handgrip 1 projecting from a body 2. The body 2 includes an elongate chamber 3 having a first end 4 shaped as an inlet for a hot melt glue stick 5 and a second end 6 equipped with a nozzle 7 for dispensing molten glue. The applicator also includes mechanical means (not shown) actuated by pressure on a trigger 8 to push the glue stick 5 progressively into the chamber 3 and electrical heating means disposed around the chamber 3 to melt the material of the glue stick 5 contained in that chamber. The aforementioned means are disposed in a volume 9 around the chamber 3 and are supplied with power by an external electrical power supply (for example the mains electrical supply) via an electrical cable 10 passing through the handgrip.
The drops of molten glue that emerge from the nozzle 7 can be used to assemble parts to each other or to supports, for example to fix moldings or claddings to walls, ceilings, etc.
The heating means incorporated in applicators of the above kind are essentially of two types at present. In the first, they comprise a cylindrical cartridge filled with a material constituting an electrical resistance and having a positive temperature coefficient. The cartridge is supplied with electrical power and is mounted in a metal sheath through which the chamber 3 passes and heats the sheath by thermal conduction. Because of the positive temperature coefficient of the material contained in the cartridge the resistance of the material increases with its temperature, which regulates the electrical heating current.
The use of a cartridge of the above kind is advantageous because of its low cost, its compact overall size and the regulation function that it assures. On the other hand, applicators equipped with this cartridge have long warm-up times (around ten minutes) and limited heating power. Furthermore, as the cartridges are supplied by specialist manufacturers the designer of the applicator is restricted in choice by the nominal performance of the cartridge selected by the manufacturer, in particular the maximal heating temperature.
Another type of heating means currently employed in the aforementioned applicators takes the form of an insulated electrical resistance in the form of a filament wound around the chamber 3 on a metal sheath through which the chamber passes. With this technology warm-up times are shorter (around five minutes), the power of the device can be varied by appropriately calibrating the resistance employed and the maximal heating temperature can be controlled by a thermostat (for example a bimetallic strip).
On the other hand, the resistance must be associated with thermal and electrical insulation means that increase the price of the applicator. When the resistance incorporates a glass filament, the latter eventually becomes fragile due to the variations in thermal flux to which it is subjected. It is more difficult to obtain accurate regulation of the heating current than with the aforementioned cartridges and the assembly and the adjustment of the applicator are relatively labor intensive, which increases the price of the applicator.
In both types of heating means described hereinabove the whole of a sheath is heated, not just the glue to be melted. A significant portion of the electrical energy consumed by the heating means is therefore dissipated elsewhere than in the glue.
Accordingly, an aim of the present invention is to provide a glue applicator that does not have the above-mentioned drawbacks of the prior art applicators and which in particular has a very short warm-up time, a high heat efficiency and a high molten glue flowrate.
Another aim of the present invention is to provide an applicator of the above kind that is reliable, that is electrically safe for the user and that has a low unit cost of manufacture.